Author(s): Chuchu Zhang, Amanda Paulson, Tom Dallman

Lesson Overview

Grade level(s):

Subjects(s):

Biology/Life Science

Topic:

Nervous system, Action potential

Big ideas(s):

Outside stimuli excite sensory neurons and create action potential. Neurons then pass on this signal, in the form of electricity, to the brain for us to "feel" the sensation. At the same time, neurons in the brain also generate action potential and pass down to the motor neurons to initiate a response.

Time needed:

1hr 20 min

Author Name(s):

Chuchu Zhang, Amanda Paulson, Tom Dallman

Summary:

The lesson has two parts: part one introduces the nerve circuitry for somatosensation and demonstrates the nature of neuronal signaling - electricity; Part two explores the concept of an action potential.

Prerequisites for students:

Students should be familiar with:

What an ascending pathway is, its components (including sensory neurons, secondary neurons etc.) and what it does

What a descending pathway its function

The concept that neurons talk to each other by neuron transmitters and transmit signals by electricity

The basic concept of an action potential

Learning goals/objectives for students:

- Students will understand that neuronal signaling is how we sense and respond to environmental stimuli.

- Students will understand how a painful sensation is transmitted to the brain that in turn triggers a signal to the motor neurons.

Content background for instructor:

The following lesson can meet the Middle School Next Generation Science Standards with some adjustments (simplify the background).

A neuron is a specialized cell with a cell body and consists of dendrites (thin protrusions) and an axon that can extend as far as a meter in humans. Their main function is to send electrical signals by connecting to other neurons via synapses. The nervous system comprises the central nervous system (brain and spinal cord) and the peripheral nervous system (specialized neurons such as sensory neurons, motor neurons and interneurons, and ganglia).

The somatosensory system incorporates several distinct “senses” or modalities and submodalities including: the tactile senses (sensing touch, pressure, vibration), thermoception (sensing temperature), proprioception (detection of body position in space), kinesthesis (sensing body movement), nociception (pain stimuli) etc. Its main function is to inform the body about the external environment and the movement of its own body parts.

Somatosensory receptors are specialized neuronal or epithelial cells; for example, neuronal photoreceptors are adapted to sensing light, while epithelial auditory hair cells have mechanoreceptors to detect air pressure. They detect and transmit sensory stimuli (pain, itch, touch, temperature etc.) through the somatosensory system. The sensory neurons that comprise the somatosensory system are classified into primary, secondary and tertiary neurons. The primaryneurons are located in the dorsal root ganglia (a cluster of neuron cell bodies in the peripheral nervous system) and connect somatosensory receptors (on the skin, muscle or joint) to the central nervous system (spinal cord). Secondary neurons of the spinal cords (or brainstem) relay the sensation to the cerebellum or the thalamus. Tertiary neurons extend from the cerebellum/thalamus (depending on the type of sensation) to the parietal lobe of the cerebral cortex. The parietal lobe (specifically, the postcentral gyrus) is the site of primary somatosensory information processing. Proprioception stimuli are processed in the cerebellum instead.

Afferent neurons carry the signal towards the central nervous system, while efferent neurons (also known as motor neurons) carry the impulse away from the brain towards a target organ or tissue (ie: muscles).

In the case of a cut to the hand, noniceptors within the skin detect damage caused by the intense painful stimulus and sends a signal up the afferent neuron pathway (also called an ascending pathway) through the spinal cord to the pain-processing region in the brain. The signal takes the form of a series of action potentials that fire repeatedly depending on the intensity of the pain. The brain reacts through multiple responses, including the autonomic nervous system (“fight or flight”) and by stimulating the motor neurons. The descending pathway travels from the somatosensory cortex to the spinal cord, ending in the motor nerves in the muscle of the hand and arm. The nerves stimulate the muscles to contract and move the arm/hand away from the source of pain. The descending pathway will also inhibit the ascending pathway to provide pain relief (in the form of neurotransmitters such as endorphins).

When a neuron receives an impulse, a depolarizing current occurs (which moves the resting potential positively). When the current reaches a threshold of -55 mV, the neuron “fires” an action potential. At this threshold, sodium channels open, letting a flood of Na+ ions into the cell and depolarizing the cell. Soon after, the sodium channels close as the potassium channels open, letting K+ ions out of the cells, reversing the polarization. Once stabilized, the cell returns to its resting potential. On a graph, an action potential looks like a sharp spike, as the potential rises and then falls rapidly. The action potential is propagated along the axon of a neuron and is transmitted to a connecting neuron. At the synapse, the synaptic neuron releases neurotransmitters to the connecting neuron, inducing ion channels to open and triggering a new action potential in the post-synaptic neuron.

The following investigation is based on “Microstimulation of Neurons and Muscles” experiment at the Backyard Brains website. Nerves from a cockroach or cricket leg are stimulated by electrodes to observe how an electric impulse can cause the leg to twitch. These direct electrical stimulations to the nervous system have a long historical precedent, and today, they are used to treat conditions such as Parkinson’s (deep brain stimulation), blindness (retina), in pain management and even muscle rehabilitation.

In the second part of the lesson, students come up with an interpretive dance (or another artistic medium) to model an action potential firing across a membrane. By discussing in a group what an action potential represents and then interpreting it to an audience, the students internalize the concepts taught in lecture.

Lesson Implementation / Outline

Introduction:

Discussion:

ASK: How does our nervous system carry message from a wound in the hand to the brain? (Assess prior knowledge)

Review the ascending and descending pathways for pain:

Emphasize that the mechanical/chemical/temperature stimulus is turned into electrical signals passed down a series of neurons in the form of action potential to the brain, and results in muscle movement.

Activity:

Part One: Demonstration of cricket "dancing" by electrical stimulation.

The activity can be done as a demonstration or in groups of students. BackyardBrains “Spikerbox” experiment is also recommended as an introduction to neural impulses (“spikes”). (Expanded and detailed protocol and demo video from Backyard Brains.)

PROCEDURE

- Take a cricket (or cockroach) and place it on ice. This will stun the cricket.

- Using sharp scissors, cut off one of the legs (with attached coxa, femur and tibia) and place it on cardboard. A cockroach can grow back its leg if young, while a cricket’s will not. They can both survive with a missing leg.

- Plug the stimulation cable in the headphone jack of a smartphone or computer.

- Place needles in the cricket leg. Attach the clips from the stimulation cable to the needles.

- Play a song with a lot of bass. Adjust the volume to see the leg “twitch” with the bass line. The electrical impulses from the gadget are used to stimulate the muscles of the cricket leg.

- Compare using a song with more treble. Can the leg still move? What other parameters can be adjusted? Change the position of the needles, play different types of music, different volumes etc.

- Optional: use the frequency generator program to control the frequency and amplitude of the tone.

DISCUSSION

ASK: What can you conclude about the nature of the impulse?

(The electricity from the computer/phone is the same in nature as the electricity generated in neurons. How can the signal translate to movement? The descending pathway results in motor neurons inducing muscle contractions.)

Discuss the potential of using electricity to control insects, animals, robots or even human parts.

ASK: What are some medical applications?

Part Two: Interpreting an Action Potential

1. Explain or review the action potential. (Emphasize how the different ion channels function during an action potential.)

Place students in groups of 10 and have them discuss the steps in an action potential. The goal of the activity is to come up with an interpretive dance, a skit, a poem or a song to illustrate an action potential in action.

Requirements:

- Time limit is 5 min

- The art piece must contain the key ion channels (sodium and potassium channels); demonstrate their gates opening and closing; and include the key ions (potassium and sodium at least)

Funded in part by by the National Center for Research Resources and the Office of Research Infrastructure Programs (ORIP) of the National Institutes of Health through Grant Number R25 OD011097 and by an undergraduate science education award from the Howard Hughes Medical Institute